Recently, magneto-optical memory devices such as magneto-optical disks have been viewed with interest as rewritable high density and large capacity memory devices. Especially, a demand for a magneto-optical memory device, whereon a so-called direct-overwriting operation can be carried out without erasing the previously recorded information, is increasing. In order to meet the demand, some methods for direct-overwriting have been proposed, which are roughly classified into the following two methods: the magnetic field modulation method and the light intensity modulation method.
An overwriting operation through the magnetic field modulation is carried out in the following way. A light beam of a constant intensity is projected on a predetermined recording area of the magneto-optical recording medium so as to raise the temperature of the recording layer of the magneto-optical recording medium, and an external magnetic field which reverses in response to the recording information is applied onto the recording area. As a result, the magnetization direction in the recording layer of the recording medium is reversed in response to the recording information, thereby enabling a direct-overwriting.
The representative recording methods by the light intensity modulation are the method which adapts the exchange coupled magnetic doublelayer film as disclosed in the technical digest of the 34th Meeting by the Applied Physics Society in 1987, page 721 (28p-ZL-3), and the method in which the demagnetizing field is utilized (Appl. Phys. Lett., 49(1986) 473-474). In the above methods, in recording, an external magnetic field of a constant intensity is applied onto the magneto-optical recording medium.
In the first method, a magneto-optical recording medium composed of an exchange-coupled doublelayer film is used, wherein a recording layer made of a magnetic substance having a large coercive force at room temperature and a low Curie temperature and a subsidiary recording layer made of a magnetic substance having a small coercive force at room temperature and a high Curie temperature are laminated. In this method, prior to overwriting, an initializing magnetic field larger than coercive force of the subsidiary recording layer at room temperature is applied onto the magneto-optical recording medium so as to arrange the magnetization direction in the subsidiary recording layer in one direction. Thereafter, with the application of the magnetic field having a magnetization in an opposite direction to the initializing magnetic field onto the magneto-optical recording medium, the temperature of the magneto-optical recording medium is raised to the vicinity of the Curie temperature of the subsidiary recording layer. Then, a direct-overwriting can be carried out by modulating the intensity of the laser beam to be projected onto the recording medium in response to information to be recorded between High level for reversing the respective magnetizations in the recording layer and the subsidiary recording layer and Low level for raising the temperature of the recording medium only up to the vicinity of the Curie temperature of the recording layer so as to prevent the magnetization in the subsidiary recording layer to be reversed, i.e., so as to arrange only the magnetization direction in the recording layer in the magnetization direction of the subsidiary recording layer.
On the other hand, in the second method, a magneto-optical recording medium having a singlelayer structure is used. In this method, a force exerted on a magnetic wall formed on the boundary between the recorded magnetic domains so as to make larger or smaller the magnetic domain is controlled by adjusting only the intensity of the laser beam to be projected. Consequently, by making smaller the size of the recording bits, the recording bits disappear (are erased), and by making larger the size of the recording bit, the direct overwriting is enabled. More concretely, by projecting a laser beam of a constant intensity, a recording bit (magnetic domain) is formed by a laser pulse with a predetermined time interval, and by projecting a laser pulse with a shorter time interval than the laser pulse onto the recording bit, the recording bit is erased.
However, in the above recording method by the magnetic field modulation, since the magnetization direction in the external magnetic field is required to be switched at high speed, the magnetic head must be made smaller so as to make smaller the inductance. Moreover, the magnetic head must be set closer to the magneto-optical recording medium, and thus the magnetic head may be brought in contact with the magneto-optical recording medium, thereby presenting the problem that the magneto-optical recording medium is scarred or the magnetic head is crushed. Namely, non-contact recording and reproducing of information may not be ensured although it is the main feature of the magneto-optical recording medium. Additionally, in order to set the magnetic field to be reversed at high speed, the drive circuit of the magnetic head must be made complicated, thereby increasing the cost of the magneto-optical recording device.
Moreover, in the method by the light intensity modulation using the exchange coupled magnetic doublelayer film, since the exchange coupled force which greatly affects the structure of the magneto-optical recording medium is used, it is difficult to set each magnetic layer to have predetermined magnetic properties especially in a large area. Moreover, besides the reproducing laser power, additional two powers of high level and low level which are used in overwriting are required to be controlled. Thus, the structure of the device becomes complicated when the above method was adapted.
In the magneto-optical recording method utilizing the demagnetizing field, the magneto-optical recording medium of a singlelayer structure is used. Thus, the magneto-optical recording medium must be strictly designed for the conformity of various properties, such as for controlling the magnetic wall energy which affects the overwriting performance, high signal quality (large magnetic Kerr rotation angle) and high recording sensitivity (low Curie temperature), etc. As described, since an acceptable range of the magnetic property of the magneto-optical recording medium is narrow, in practice, the recording magnetic domain by overwriting does not disappear easily. Although the remaining magnetic domain is small, it reduces the erased ratio. In overwriting, the previous recording magnetized state must be known, and thus a complete overwriting by a single light beam cannot be ensured.